1. Field of the invention
The present invention relates to a method for manufacturing a metallic porous sheet comprising a plurality of porous sheet layers and the metallic porous sheet manufactured by the method. It is important that the skeleton of the metallic porous sheet be unchanged and the percentage of the porous area thereof per area not be varied even though tensile force is applied thereto. The metallic porous sheet is used as electrode plates of batteries such as a nickel cadmium battery, a lithium battery, a fuel battery and the like by applying the powder of an active substance uniformly to the metallic porous sheet. Normally, the active material is successively applied to a metallic porous sheet by applying tensile force to the metallic porous sheet when an electrode plate of battery is manufactured.
2. Description of the related art
Heretofore, a plated metallic porous sheet comprising a three-dimensional net-shaped foamed sheet consisting of, for example, polyurethane sponge, a nonwoven fabric sheet or a mesh (net) has been used as the anode and cathode plates of batteries such as a nickel cadmium battery, a lithium battery, a fuel battery and the like.
In manufacturing the electrode plate of a battery using the above-described metallic porous sheet, it is required that the metallic porous sheet have a predetermined tensile strength in order to successively apply an active substance such as nickel hydroxide to the metallic porous sheet. It is necessary for the metallic porous sheet to have tensile strength of more than 3 kg/2 cm, preferably 7 kg/2 cm so that the metallic porous sheet to which tensile force is successively applied can be prevented from being extended and the skeleton thereof is unchanged to keep the percentage of the porous area thereof per area uniform even though tensile force is applied thereto.
The tensile strength of the known metallic porous sheet cannot exceed 3 kg/2 cm unless the amount of a metal deposited thereon is more than 500 g/m.sup.2 .about.600 g/m.sup.2. If the amount of the metal deposited on a metallic porous foamed sheet is less than 500 g/m.sup.2, openings thereof are deformed when tensile force is successively applied to a metallic porous foamed sheet and if the amount of a metal deposited on a metallic porous sheet is less than 300 g/m.sup.2, any type of metallic porous sheets will be broken when tensile force is successively applied thereto. Thus, it has been impossible to successively apply tensile force to a sheet consisting of a metallic porous sheet when applying the active substance to the metallic porous sheet.
Accordingly, it is necessary to plate the metallic porous sheet as thick as 500 g/m.sup.2.sub..about. 1,000 g/m.sup.2 so that the metallic porous sheet has the required tensile strength, which means that a great amount of a metal is consumed. Hence, it a high manufacturing cost results. Further, is not easy to plate the metallic porous sheet in a uniform thickness when thickly plating the porous sheet so as to obtain the required tensile strength. When the metallic porous sheet is used as the electrode plate of a cylindrical battery, normally, the metallic porous sheet is mounted in the battery with the curvature of the curved surface thereof small. If the curvature is, for example, 3 mm, a crack is likely to occur in the metallic porous sheet in the case where a large amount of a metal is deposited thereon. In particular, openings (lattice) of a metallic porous sheet comprising a sponge are likely to become cracked or damaged. Hence, an unfavorable electrical conductivity results, which leads to the deterioration of a battery. When a swirled metallic porous sheet is mounted in a battery, needless to say, the outer portion thereof extends more than the inner portion thereof because the metallic porous sheet has a certain thickness. Thus, cracks or damage occurs in the outer portion thereof. When a swirled metallic porous nonwoven fabric sheet is mounted in a cylindrical battery, a short fiber of a metal projects therefrom. Although the metallic nonwoven fabric sheet is swirled in the cylindrical battery so as to separate the anode and cathode thereof from each other by a separator, the separator is damaged by the projection of the short fiber of the metal which is caused by the crack of the outer portion of the metallic porous nonwoven fabric, thus causing a short circuit. This is a serious drawback.
In addition, when only foamed sponge sheets or only nonwoven fabric sheets are successively transported to form a metallic porous sheet by plating a base plate, tensile force is unavoidably applied to the metallic porous sheet. The skeletons of the foamed sponge sheet and the nonwoven fabric sheet are easily deformed when tensile force is applied thereto. Consequently, the diameters of openings thereof are deformed, namely, the undesirable change of the ratio of the porous area thereof to the nonporous area. Thus, the percentage of the porous area of the metallic foamed sheet or the metallic nonwoven fabric sheet per area is changed. Therefore, the active substance cannot be applied thereto. In the case of a secondary battery, charging and discharging are repeated several hundreds of times through the electrodes. Accordingly, if the skeleton of the foamed sponge sheets and the nonwoven fabric sheets are nonuniform and the powder of the active substance is not uniformly applied to the metallic porous sheet, the electrodes is not uniformly electrified. Thus, the performance of the battery is undesirable.